Degrees of general hardness (dGH) is a unit of water hardness, specifically of general hardness. General hardness is a measure of the concentration of divalent metal ions such as calcium and magnesium (Ca2+, Mg2+) per volume of water. Specifically, 1 dGH is defined as 10 milligrams (mg) of calcium oxide (CaO) per litre of water, which is equivalen... Found on http://en.wikipedia.org/wiki/DGH